A member that transmits heat generated from a heat generation portion of an electrical/electronic apparatus to a heat dissipation member must have excellent thermal conductivity and electrical insulating properties. As a member that satisfies such requirements, a thermal conductive sheet in which an inorganic filler is dispersed in a cured material of a thermosetting resin is in wide use. As the inorganic filler that is used in the thermal conductive sheet, alumina, boron nitride, silica, aluminum nitride and the like can be used. Among these, boron nitride is suitably used in the thermal conductive sheet because it has excellent chemical stability in addition to thermal conductivity and electrical insulating properties, and further is harmless and relatively cheap.
Boron nitride has the same molecular structure as that of graphite. Further, generally commercialized boron nitride has a scaly crystal structure as shown in FIG. 5. Boron nitride has a thermal anisotropy such that the thermal conductivity is high in an a-axis direction (face direction) and low in a c-axis direction (thickness direction), and the thermal conductivity in an a-axis direction of the crystal is said to be several to several tens of times that in a c-axis direction. Further, crystal growth of boron nitride is more preferential in the a-axis direction than in the c-axis direction, and the shape of a primary particle is broad in a (002) face parallel to the a-axis direction and narrow in a (100) face parallel to the c-axis direction. Therefore, the (002) face is called a stacking face and the (100) face is called an edge-face. Further, as obvious from the molecular structure of boron nitride shown in FIG. 6, there are functional groups such as hydroxyl groups, amino groups and the like on a face of a particle of boron nitride, and these mainly form covalent bonds with boron atoms on the edge-face of boron nitride. Thus, boron nitride has a feature that its affinity with organic solvents and resins becomes high due to the presence of these functional groups.
Further, since the thermal conductivities of the inorganic fillers such as boron nitride and the like are larger than that of the resin, in order to improve the thermal conductivity of the thermal conductive sheets, thermal conductive sheets in which the content of the inorganic filler is increased have been developed.
For example, Patent Document 1 proposes a thermal conductive sheet of which thermal conductivity is improved by incorporating 60% by volume or more in total of a spherical filler of which the average particle size is specified and a non-spherical filler of which the average length and aspect ratio are specified.
Further, Patent Document 2 proposes a thermal conductive sheet of which thermal conductivity and electrical insulating properties are simultaneously improved by combining two kinds of secondary particles of boron nitride which are different in cohesive strength as the inorganic filler, and by setting the content of the inorganic filler to 40% by volume to 80% by volume.